This invention is directed to coating compositions particularly useful for coating plastics such as polystyrene foam substrates, and more particularly relates to a water-dispersed coating based on a polymeric mixture of epoxy resin and functional acrylic copolymer adapted to crosslink and cure at ambient room temperatures.
Coatings for plastic substrates such as polystyrene foam suffer from numerous deficiencies. For instance, solvent dispersed polymers ordinarily physically attack the plastic substrate. In particular, the solvent quickly attacks polystyrene foam substrates thereby rendering solvent dispersed coating compositions unusable. With water based coatings, a typical problem is poor adhesion or no adhesion to plastic substrate surfaces. Adhesion of conventional water based or water-dispersed coatings to polystyrene foam is poor and especially poor to high impact polystyrene substrates.
It now has been found that certain combinations of water dispersed acrylic copolymers containing both carboxy and tertiary amine functionality in combination with epoxy resin provide a highly desirable polymeric composition adapted to be water-dispersed and advantageously adheres to plastic substrates and especially polystyrene foam and high impact polystyrene substrates. The composition can be crosslinked and cured at ambient temperatures and provides excellent adhesion without detrimentally affecting the plastic or polystyrene substrate. The coating does not require metallic alkoxide additives or chelating agents as provided in a solvent based acrylic/epoxy coating disclosed in U.S. Pat. No. 4,558,076. The composition of this invention is water dispersed and unlike the prior art patents in that the composition of this invention includes both carboxyl and tertiary amine functionality in the same acrylic copolymer whereby the acrylic copolymer is self neutralizing and water reducible along with the epoxy resin. The advantages include low VOC, no solvent attack plastic substrates, low temperature cure, and good adhesion to plastic substrates. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples. cl SUMMARY OF THE INVENTION
Briefly, the invention pertains to a water dispersed, ambient cure coating composition particularly useful for coating plastic substrates. The coating contains a polymeric binder comprising an epoxy resin mixed with a functional acrylic copolymer containing both carboxy and tertiary amine functionality to render the acrylic copolymer water dispersible along with epoxy resin. The polymeric binder comprises on a weight basis between 20% and 99% acrylic copolymer with the balance being epoxy resin.